The pressurized-water type nuclear power station (atomic power generation facility) is a reactor facility which heats pressurized water (light water with high pressure) that is a primary coolant to 300° C. or higher with thermal energy generated by a nuclear fission reaction, boils a light water of a secondary coolant with a steam generator to eventually convert the light water into steam of high temperature and high pressure, and rotates a turbine generator by using the steam to generate an electric power. This pressurized-water type reactor is used for large-sized plants such as a nuclear power station, and small plants such as a nuclear vessel (atomic-powered ship).
In various plants that include the above described pressurized-water type atomic power generation facility and have a boiler, a steam generator, a heat exchanger and/or the like, in which high-temperature water circulates, it becomes a big problem that ions elute from the metal of the structural material or the structural material itself corrodes. The elution of the metal ions is a representative phenomenon occurring in the high-temperature water, and the elution causes the corrosion of structural members of pipes and equipments, including the structural material, and eventually gives various influences such as an operational problem and the increase of maintenance frequency, on the plant.
In addition, the eluted metal ions from the structural material and the like adhere to and deposit on a surface of the pipes in the system, or a high-temperature site of the steam generator and the like, as an oxide, and there is a possibility that impurities form a highly concentrated state, in a narrow portion such as a crevice portion between a heat transfer tubing and a tube-support-plate in a heat exchanger. The impurities also may form an ion-enriched water having strong acidity or strong alkalinity according to the ion balance, and further cause remarkable corrosion.
A phenomenon of corrosion cracking in the structural material is also confirmed which is caused by such a phenomenon and a rise of an electrochemical potential due to the oxide which adheres to the surface. Heat transfer also decreases due to the adhering oxide, and accordingly it is needed to remove the oxide on the structural material by chemical cleaning or the like periodically with a high frequency.
On the other hand, there has been a high possibility in recent years that the thickness of a carbon steel pipe decreases due to a wall-thinning phenomenon of the pipe and such an accident that the pipe is ruptured also occurs. Thus, the elution, the corrosion phenomenon and the like of the metal are accumulated with time during a plant operation in a long period of time, and potentially show a possibility of suddenly erupting into a disaster at some point when the accumulated amount has reached to a durable limit.
Furthermore, the above described corrosion rate is accelerated depending on a shape of a structural site, and a phenomenon which is difficult to be predicted may occur. For instance, in a piping system in which many equipments such as an orifice and a valve are used, erosion or corrosion is caused by the flow of a fluid of high temperature such as a cooling water which passes through the inner space at a high speed. In order to avoid such a problem, various corrosion mitigation methods including a water chemistry control have been conventionally implemented in various plant systems.
For instance, in the secondary cooling system of a thermal power station and a pressurized-water type nuclear power station, such measures are taken as to control a pH in a cooling water by injecting ammonia or hydrazine, thereby decrease the elution of iron from the inside of the system and prevent the inflow of the iron component to the steam generator (Patent Literature 1).
Furthermore, in order to eliminate the enrichment of alkaline components in the crevice portion, various water chemistry controls have been implemented in an actual plant, such as the control of an Na/Cl ratio, the control of chloride ion concentration for decreasing an influence of a chlorine ion on corrosion, and the control of dissolved oxygen concentration (Patent Literature 2). In recent years, a water chemistry control method is also adopted which uses improved chemicals such as ethanolamine and morpholine.
As described above, various technologies for controlling the water chemistry have been proposed as an improved proposal, in addition to the measures which have been already implemented in the actual plant, such as reductions of the corrosion of pipes, the adhesion and deposition of an oxide and the like, and the enrichment of eluted components in the crevice portion. As for the improvement of the chemicals to be injected, for instance, there is a method of using an organic acid such as tannic acid and ascorbic acid as an oxygen scavenger (Patent Literature 3).
In addition, as for the water chemistry control method, there are proposed an operation method of controlling a molar ratio of all cations/SO4 (Patent Literature 2), a method of introducing at least one of a calcium compound and a magnesium compound into feed water to a steam generator for a reactor so that the ion concentration becomes 0.4 to 0.8 ppb (Patent Literature 2), and the like.
Thus, the measures of suppressing corrosion and elution by water chemistry control with the use of the chemicals are widely implemented under present circumstances as a measure of preventing the corrosion and elution of a plant structural material. However, such a technology is desired which can operate the plant without controlling a water chemistry of the cooling water by injecting the chemicals, from the viewpoints of the complexity of operation management, an operation cost and the safety.